String.get s n returns character number n in string s.
The first character is character number 0.
The last character is character number String.length s - 1.
You can also write s.[n] instead of String.get s n.

Raise Invalid_argument"index out of bounds"
if n is outside the range 0 to (String.length s - 1).

val set : string -> int -> char -> unit

String.set s n c modifies string s in place,
replacing the character number n by c.
You can also write s.[n] <- c instead of String.set s n c.
Raise Invalid_argument"index out of bounds"
if n is outside the range 0 to (String.length s - 1).

String.blit src srcoff dst dstoff len copies len characters
from string src, starting at character number srcoff, to
string dst, starting at character number dstoff. It works
correctly even if src and dst are the same string,
and the source and destination chunks overlap.
Raise Invalid_argument if srcoff and len do not
designate a valid substring of src, or if dstoff and len
do not designate a valid substring of dst.

Return a copy of the argument, with special characters
represented by escape sequences, following the lexical
conventions of Objective Caml. If there is no special
character in the argument, return the original string itself,
not a copy.

val index : string -> char -> int

String.index s c returns the position of the leftmost
occurrence of character c in string s.
Raise Not_found if c does not occur in s.

val rindex : string -> char -> int

String.rindex s c returns the position of the rightmost
occurrence of character c in string s.
Raise Not_found if c does not occur in s.

val index_from : string -> int -> char -> int

Same as StringLabels.index, but start
searching at the character position given as second argument.
String.index s c is equivalent to String.index_from s 0 c.

val rindex_from : string -> int -> char -> int

Same as StringLabels.rindex, but start
searching at the character position given as second argument.
String.rindex s c is equivalent to
String.rindex_from s (String.length s - 1) c.

val contains : string -> char -> bool

String.contains s c tests if character c
appears in the string s.

val contains_from : string -> int -> char -> bool

String.contains_from s start c tests if character c
appears in the substring of s starting from start to the end
of s.
Raise Invalid_argument if start is not a valid index of s.

val rcontains_from : string -> int -> char -> bool

String.rcontains_from s stop c tests if character c
appears in the substring of s starting from the beginning
of s to index stop.
Raise Invalid_argument if stop is not a valid index of s.

val uppercase : string -> string

Return a copy of the argument, with all lowercase letters
translated to uppercase, including accented letters of the ISO
Latin-1 (8859-1) character set.

val lowercase : string -> string

Return a copy of the argument, with all uppercase letters
translated to lowercase, including accented letters of the ISO
Latin-1 (8859-1) character set.

val capitalize : string -> string

Return a copy of the argument, with the first character set to uppercase.

val uncapitalize : string -> string

Return a copy of the argument, with the first character set to lowercase.

The comparison function for strings, with the same specification as
compare. Along with the type t, this function compare
allows the module String to be passed as argument to the functors
Set.Make and Map.Make.